<html>
<head>
<base href="https://bugs.llvm.org/">
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [Clang-optimization] wrong code for all optimizations with an union assignment in a loop"
href="https://bugs.llvm.org/show_bug.cgi?id=35568">35568</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>[Clang-optimization] wrong code for all optimizations with an union assignment in a loop
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>yangyibiao@nju.edu.cn
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>$ clang -v
clang version 6.0.0-svn320088-1~exp1 (trunk)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.2.0
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0
Candidate multilib: .;@m64
Selected multilib: .;@m64
$ cat small.c
int printf(const char *, ...);
union {
long a;
int b;
int c;
} d;
int *e = &d.c;
char f, g;
int main() {
for (; f >= 0; f--) {
long *h = &d.a;
*h = g;
*e = 1;
}
printf("%d\n", d.b);
}
$ clang -O0 small.c; ./a.out
1
$ clang -O1 small.c; ./a.out
0
$ clang -O2 small.c; ./a.out
0
$ clang -O3 small.c; ./a.out
0
$ clang -Ofast small.c; ./a.out
0
As we can find that, when no optimization is outcome is 1. When enabling
optimization, the output is 0. When remoing the for loop in the main function,
the result is correct.
$ cat small-correct.c
int printf(const char *, ...);
union {
long a;
int b;
int c;
} d;
int *e = &d.c;
char f, g;
int main() {
// for (; f >= 0; f--) {
long *h = &d.a;
*h = g;
*e = 1;
// }
printf("%d\n", d.b);
}
$ clang -O0 small-correct.c; ./a.out
1
$ clang -O1 small-correct.c; ./a.out
1
$ clang -O2 small-correct.c; ./a.out
1
$ clang -O3 small-correct.c; ./a.out
1
$ clang -Ofast small-correct.c; ./a.out
1</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>